As any person of the art would know, a proposition has been made, particularly through the patent document WO 2011/045180, to define on a selected part of a physical medium, such as for example a plastic substrate, a matrix of M*P pixels in N colours (possibly primary colours) defining a colour coding system.
A ‘colour coding system’ is a set of N colours (possibly primary colours) that make it possible to define any visible colour when they are combined with each other. As non-limitative examples, the system may be the so-called RGB (Red, Green, Blue) system or the so-called YMC (Yellow, Magenta, Cyan) system or the so-called YMCRGB (Yellow, Magenta, Cyan, Red, Green, Blue) system.
The matrix comprises M ‘horizontal’ lines comprising P pixels each in the N colours (possibly primary colours) successively according to a selected order (for example Red (R) then Green (G) then Blue (B)), with the possible repetition of that order when P is greater than N (for example R G B R G B R G B). Besides, each horizontal line other than the first one comprises a first pixel with a colour identical to that of the second pixel of the previous line. That is what is called a line shift matrix.
This type of matrix constitutes a diagram or a chart that is designed to contribute to the definition of an image in colour, for example a face. In fact, when the matrix has been printed on the physical medium, for example by the ink jet method, a first transparent light-sensitive layer (for example in doped polycarbonate) may be deposited over it, wherein said layer is capable of being modified selectively by a first laser (for example of the 355 nm UV type or 532 nm green UV type) to become locally opaque white at predefined locations, then, on top of the first transparent light-sensitive layer, a second transparent light-sensitive layer (for example in carbon-doped polycarbonate) may be deposited over it, wherein said layer is capable of being modified selectively by a second laser (for example of the 1064 nm Nd-YAG type) to become locally opaque black (or in shades of grey) at predefined locations. After that, at least one exposure mask is generated from the digital data file that defines the image used for personalising the physical medium, then each exposure mask is placed very precisely on top of the second opaque light-sensitive layer and the whole is exposed successively to the light supplied by the first laser and then to the light supplied by the second laser in order to generate a reproduction of the image on the physical medium. A galvanometric scan head system may equally and advantageously be used, to distribute all the laser pulses to different positions that correspond with the locations where the mask or masks must let through the light supplied by the lasers.
One of the drawbacks of that method lies in the embodiment of the pixel matrix. That is because the pixel matrix can be made by print heads of different colours that are not located in the same position on the path of the support, leading to slight errors in the positioning of the pixels in relation to each other because of their different ink combinations to define the different colours that need to managed pixel by pixel. These errors are generally the result of the alignment of the print heads and/or the relative non-constant speed of the support under the print heads and/or the mechanical play in the transport of the support and/or the speed of ejection of ink drops in the case of an ink jet printing system (purely for information and thus not limitative). Now, these positioning errors can lead to the partial superposing of neighbouring pixels in different colours and therefore uniformity defects of the N colours (possibly primary colours) over the entire matrix.
Besides, because of these positioning errors of the colour pixels in the matrix, the pixels in different shades of grey and the white pixels (or transparent pixels) that are generated during the stage of modification in the first and second light-sensitive layers respectively on top of the corresponding colour pixels are also not correctly positioned on the latter, which leads to local intensity and/or colour and/or shade errors in the reproduced image, which degrade the perceived quality by giving rise to a moiré effect or other colorimetric effects. That degradation is all the more intense because the stages of generation of pixels in different shades of grey and transparent pixels are also carried out one horizontal line after another by the horizontal and then vertical displacement of the modification means (laser) in relation to the physical medium, and thus give rise to slight positioning errors, but which may not exceed 20% of the size of the colour pixels in order to limit the degradation of the quality of the image produced.